usb: xhci-plat: Add support to modify imod from sysfs

xhci allows interrupts to be moderated at 250ns interval;
provide an sysfs interface to modify interrupt rate from
userspace to understand throughput variation by changing imod.

Usage:
Below command will moderate interrupts at 1ms or (4000 * 250ns)
interval.
echo 4000 > /sys/devices/8a00000.ssusb/8a00000.dwc3/xhci-hcd.0.auto/
config_imod

Change-Id: I455d5a0f9a0314e86f711efd35315002b20f5343
Signed-off-by: Devdutt Patnaik <dpatnaik@codeaurora.org>
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>